First-time home buyers, long a key underpinning of the housing market, are increasingly getting left behind in the real-estate recovery.

Such buyers, typically couples in their late 20s or early 30s, have accounted for about 30% of home sales over the past year. They represented 40% of sales, on average, over the past 30 years, and accounted for more than 50% in 2009, when recession-era tax credits fueled the first-time market,...